I'll show you mine

Captain LearnCurve's depth gauge read 107', yet the water was so clear
he could see birds in the sky. His dives at Punta Perdiz on the Bahia
de Cochinos, better known to Americans as the Bay of Pigs in Cuba, would
go into the log book as two of the all-time best out of 153. As he surfaced,
the warm tropical breeze carried the cry of an anguished AutoCAD user:
"My sales manager doesn't understand me!"

First Thing First
The first thing I teach in my Engineering Graphics I class is that a photograph
or detailed pictorial drawing of a part is next to useless for engineering
purposes. To accurately convey manufacturing information, we need to use
dimensioned, annotated, multiview orthographic drawings. Those are fine
until you need to explain your design to someone who can't read a blueprint.
Sales, marketing, clients, and customers don't need to know detailed dimensions,
but they do want to see what the design looks like. They need a picture.

Fortunately, AutoCAD comes equipped with a full set of visualization
tools to display 3D models. They run the range from a simple hidden-line
removal operation (figure 1) to photorealistic renderings complete with
transparency and shadows (figure 2). This month's column introduces basic
principles to start you on the road from figure 1 to figure 2.

Figure 2. AutoCAD can also generate photorealistic
renderings of the same objects.

I'll use mechanical parts for simplicity, but the same principles apply
to any 3D model, up to and including architectural models of buildings
or complete towns.

Hide and Seek
Figure 3 shows a simple 3D model I generated using standard AutoCAD solid
modeling techniques. Confusing, isn't it? Even though it was built as
a solid model, it normally displays as a wire frame that just shows the
edges.

The first visualization command to appear in AutoCAD was Hide. To ensure
that your system's behavior matches our example, you first need to set
the following:

At the Command prompt enter the system variable D
I S P S I L H and set its value to 0 (zero).

Now, pick View | Hide to launch the Hide command. Figure 4 starts to
get a little easier to understand.

Keep in mind the following points about the Hide command.

Solids are not the only thing that react to the Hide
command. Standard lines, arcs, and text that uses AutoCAD SHX fonts
(as opposed to Windows Truetype fonts), if they have thickness, hide
objects behind them as though they were thin sheet-metal walls.

3D faces and polygon meshes hide objects that are behind them.

Circles with zero thickness behave as though they were thin-sheet
disks. If they have thickness, they hide their own internal details
and objects behind them as if they were solid cylinders.

Polylines with width and zero thickness act like thin ribbons. If
they have thickness, they hide themselves and other objects as though
they were a solid bar.

When you finish admiring the hidden view, you must Regen the drawing
(select View | Regen) to get the wire frame back before you can zoom
or pan in real time. Starting 3D Orbit automatically forces a Regen
that unhides the model.

You can edit the drawing while it is hidden.

Note the triangular patches that appear all over the curved face in figure
4. You can set the DISPSILH variable to 0 (zero) to turn them on (figure
4), or to 1 to turn them off (figure 5). This is backward from what you
might expect, but I won't bore you with the long explanation.

Figure 3. A wire frame representation of a model
of a 3D part.

Figure 4. The part is easier to understand with
hidden lines removed.

Figure 5. You can turn off the triangles
on the curved face.

Later releases of AutoCAD feature more advanced hiding and shading functionality,
so Hide is not used as much today. The only reason to bring it up now
is that if you plot a drawing with Hide Edges turned on, the plot produces
hidden objects exactly like the Hide command. In particular, note how
DISPSILH affects the presence or absence of the triangles on curved faces.

Do you see what I see?
Current AutoCAD releases have a much more powerful command called Shademode.
The Shade toolbar and the View | Shade menu picks each appear to have
seven commands below them, but these are actually just seven different
Shademode settings. That they are modes rather than specific commands
is significant.

If you select Hidden mode, the drawing looks much like figure 5 again,
except Hidden is now a "sticky" setting. As you edit, zoom,
pan, and 3D orbit the model, the "hidden" view continuously
updates in real time.

You should also note a couple of other differences from the Hide command,
and hence from hidden plots:

DISPSILH has no effect. Curved surfaces always hide
without showing the triangles.

Circles with thickness hide other objects as though they are lengths
of tubing rather than solid cylinders.

The thickness of thick (as opposed to wide) polylines is ignored.
They hide objects as though they had zero thickness.

This is a continuous mode, so it overrides the Hide command. Before you
can use the Hide command again you must set Shademode to 2D Wireframe.
The easy way to do this is by menu or toolbar pick.

It's cool in the shade
Current releases of AutoCAD go far beyond the simple hidden-line operations
we have discussed so far.

Let's start with the different shading operations. From the menu bar
pick View | Shade | Gouraud Shaded, or pick the Gouraud Shaded button
on the Shade toolbar. This produces figure 6, which is beginning to look
a little more realistic. The part is illuminated by a single light source
located above and to the left. The color of each face is determined only
by its distance from the light (farther away is darker), and there are
no shadows.

Figure 6. The part is easier
to understand when Gouraud Shaded is applied.

Figure 7. The Flat Shaded
option is faster, but the curved surface now has flat facets on it.

Once again, this is a mode of the Shademode command, so it works in real
time. You can edit, zoom, pan, snap to, and 3D Orbit the shaded model.

Besides Gouraud, three other shade modes are available. Figure 7 shows
Flat Shaded mode. Notice how a series of flats replaces the nice, smooth
curved face of figure 6. This mode is less realistic than Gouraud, but
generates more quickly on a complex part with lots of curves.

Figures 8 and 9 show the final two modes. These are Gouraud and Flat
with edges on. Notice how the part seems to be made of colored glass so
you can see the wire frame showing through.

The edges-on modes can be useful when you edit the shaded model. AutoCAD
always snaps to objects in the model, even when they are hidden or shaded,
but edges on makes it easier to snap to the correct place.

Now that you have a nice shaded image of the part, how do you show it
to someone else who doesn't have AutoCAD? Simple. Press the <Alt>
<PrintScreen> key combination. This takes a snapshot of your entire
AutoCAD window and places it on the Windows Clipboard. You can now paste
it into almost any other Windows application, such as a Word document
or PowerPoint presentation.

Paste your image into Paint first to crop it down to cut out the menu
bars, Command line area, and so on. This is how I create all the screen
shots for my articles.

Be sure to come back next time when we play with the Render command.
You'll learn how simple it can be to produce very realistic images of
your AutoCAD models.

And now for something completely differentIf you ever go to Cuba, do not take a satellite GPS receiver with
you. They x-ray your baggage when you arrive. Four people needed six pieces
of paper and 45 minutes to confiscate it on the way in, and three more
people needed four more pieces of paper and another 45 minutes to give
it back a week later on the way out. Other than that, it was a great vacation.

About the Author: Bill Fane

Autodesk Technical Evangelist Lynn Allen guides you through a different AutoCAD feature in every edition of her popular "Circles and Lines" tutorial series. For even more AutoCAD how-to, check out Lynn's quick tips in the Cadalyst Video Gallery. Subscribe to Cadalyst's free Tips & Tools Weekly e-newsletter and we'll notify you every time a new video tip is published. All exclusively from Cadalyst!Follow Lynn on Twitter